Description of the Prior Art
Prior art CMOS operational amplifiers were designed as transconductance amplifiers and also as general purpose operational amplifiers. Transconductance amplifiers are limited to driving capacitive loads only. General purpose operational amplifiers are able to drive resistive as well as capacitive loads.
Presently known operational amplifiers include output stages with limited driving capability, particularly when driving small resistive loads, because the gate-to-source voltage (V.sub.gs) of the output devices is lmited. To realize the capability of higher current driving, large size output transistors and a relatively high gate-to-source voltage are required.
When designing integrated circuits, a major objective is to use physically smaller components in order to make the circuit assembly more compact. Therefore, one goal in the implementation of integrated circuits is to use smaller size transistors than those found in conventional circuits, but yet to achieve optimum operating characteristics of the circuit with good current driving capability. However, in presently known operational amplifiers, large size transistors are employed to achieve high current driving capability for small resistive loads which is mostly due to limited gate-to-source voltage.